The US does have a homicide rate 3-10 times greater than other developed countries and a gun death rate 20-50 times greater than other developed countries, and in line with Guatemala, El Salvador etc.

Just because it’s not a strictly linear increase with the number of guns does not mean they aren’t causative.

In fact, that statistic is deliberately misleading because you can only really murder people with one gun at once, so the more guns you own, the less likely any individual gun is to be a murder weapon.